RELATIONSHIP BETWEEN OIL GLANDS AND OIL YIELDS. — 151 
Oils,’ 2nd Hdition, Baker and Smith. In practice it is 
found that the yield of oil varies within limits during the 
different seasons of the year, and it is to be expected that 
climatic and ecological conditions would also influence the 
yield to some extent. It has been found also that the yield 
of oil is higher in young foliage than when older leaves are 
used. 
A microscopical examination shows that in a number of 
secretory cavities very little oil is present. Sometimes 
only a fringe of a dark granular nature is found, whilst in 
other cases the cavity contains chiefly loose thin walled 
tissue. 
In the accompanying table the average number of glands 
_per length of 5 mm. of transverse section is given in the 
first column. In the second column, a correction is made 
for the thickness of the leaf and the mean number of glands 
per sq. mm. of cross section is shown. Inthe third column 
the relative volumes of the glands are given, assuming 
that the cavity is spherical; this is not always the case, 
but for the purposes of this comparison is a sufficiently 
accurate approximation. In the fourth column a figure is 
given which is the product of the relative capacity of the 
gland and the number per sq. mm. of section. This figure, 
which gives a basis for comparison of the possible oil yield 
of the different species, can be compared with the oil yield 
which has been obtained by steam distillation, and shown 
in column 5. 
It is seen that although the theoretical figures for certain 
species with small yields e.g., H. corymbosa, E. coryno- 
calyx, E. botryoides, EK. obtusifiora, and H. nigra, agree 
fairly well with the actual yields, in the majority of species 
this is not so, and there is such a wide variation, particu- 
larly in H. aggregata and HE. rubida, that it is impossible 
